[test-improver] Improve tests for httputil package

[github-actions]

Test Improvements: httputil_test.go

File Analyzed

• Test File: internal/httputil/httputil_test.go
• Package: internal/httputil
• Lines of Code: +62 lines added

Improvements Made

1. Increased Coverage

• ✅ Added errorResponseWriter mock http.ResponseWriter to simulate write failures and short writes
• ✅ Added test for write-error path (w.Write returns an error — previously unreachable)
• ✅ Added test for short-write path (w.Write returns n < len(data) — previously unreachable)
• Previous Coverage: WriteJSONResponse at 78.6% → package at 88.9%
• New Coverage: WriteJSONResponse at 100% → package at 100.0%
• Improvement: +11.1 percentage points (package), +21.4pp for WriteJSONResponse

2. Better Testing Patterns

• ✅ Uses require.NotPanics to verify graceful error handling
• ✅ Mock writer verifies actual byte-count and header state after error conditions
• ✅ Follows existing subtests-inside-TestWriteJSONResponse pattern

3. Cleaner & More Stable Tests

• ✅ Mock is self-contained in the test file (no new dependencies)
• ✅ newErrorResponseWriter / newShortResponseWriter constructors keep test cases readable
• ✅ Tests are deterministic and free of external dependencies

Test Execution

All httputil tests pass:

=== RUN   TestWriteJSONResponse/write_error_does_not_panic
--- PASS: TestWriteJSONResponse/write_error_does_not_panic (0.00s)
=== RUN   TestWriteJSONResponse/short_write_does_not_panic
--- PASS: TestWriteJSONResponse/short_write_does_not_panic (0.00s)
...
PASS
coverage: 100.0% of statements
ok      github.com/github/gh-aw-mcpg/internal/httputil  0.005s  coverage: 100.0% of statements

Note: A pre-existing failure in internal/config.TestFetchAndFixSchema_NetworkError (unrelated to this PR) exists on main and is not caused by these changes.

Why These Changes?

WriteJSONResponse had two error-handling branches that were never tested: the path where w.Write returns an error, and the path where it performs a short write. These are realistic failure modes in production (e.g., broken connections, buffers full). The missing coverage was identified via go test -coverprofile, and a small mock ResponseWriter was the minimal addition needed to exercise both paths without introducing new dependencies.

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